Silica soot particles are a by-product of high purity fused silica glass making processes. For example, conventional chemical vapor deposition (CVD) processes for making optical fiber preforms, such as outside vapor deposition (OVD) and vapor axial deposition (VAD) processes, often utilize only a portion of the starting raw material due to limitations in the deposition efficiency of the processes. Use of the resulting “waste” silica soot in forming optical quality glass could, therefore, potentially result in significant raw material cost savings.
Accordingly, different methods have been devised to utilize otherwise unutilized silica soot in the production of optical quality glass. These methods, which include, for example, sol-gel (and other “wet”) processes, can suffer from a variety of drawbacks including expensive, complicated, and/or time consuming processing conditions and equipment, and may result in soot compacts with less than desirable properties such as unacceptable variability with respect to compact density and geometry. These less than desirable properties adversely affect compact strength and can result in cracking, breaking or other types of soot compact failure.
According to one method, soot compact strength may be improved by increasing the water content in a silica soot sample. However, as water is volatile, and can be difficult to distribute throughout the silica soot sample, even an initially well distributed water supply is subject to drying and loss of cohesive strength over time. As a result of compaction that occurs as the meniscus force of the drying draws particles together, agglomerates that retain high density in the silica soot are formed. These agglomerates adversely affect soot compact strength and may lead to the formation of pores in the resultant glass.
Organic additives may also be effective in strengthening soot compacts. However, removal of organic additives from high purity silica soot can be difficult to achieve and may necessitate additional high temperature processing steps. If unsuccessfully removed, the organic additives may become trapped in the silica soot through consolidation and may form pores in the resultant glass. Still other additives that may improve soot compact strength require a high temperature active oxidation or chlorination to be removed from the silica soot. At the temperatures required for removal, these additives may interact with the silica surface of the silica soot and initiate irreversible crystallization.